Adaptive x-ray control

ABSTRACT

A method, apparatus and system for measuring the signal-to-noise ratio of x-ray images and to adaptively control x-ray exposure in response to image quality and patient movement.

TECHNICAL FIELD

Embodiments of the invention relate to medical imaging and, inparticular, to the control of x-ray exposure during medical imaging.

BACKGROUND

Radiosurgery and radiotherapy systems are radiation treatment systemsthat use external radiation beams to treat pathological anatomies (e.g.,tumors, lesions, vascular malformations, nerve disorders, etc.) bydelivering a prescribed dose of radiation (e.g., x-rays or gamma rays)to the pathological anatomy while minimizing radiation exposure tosurrounding tissue and critical anatomical structures (e.g., the spinalchord). Both radiosurgery and radiotherapy are designed to necrotize thepathological anatomy while sparing healthy tissue and the criticalstructures. Radiotherapy is characterized by a low radiation dose pertreatment, and many treatments (e.g., 30 to 45 days of treatment).Radiosurgery is characterized by a relatively high radiation dose inone, or at most a few, treatments.

In both radiotherapy and radiosurgery, the radiation dose is deliveredto the site of the pathological anatomy from multiple angles. As theangle of each radiation beam is different, each beam can intersect atarget region occupied by the pathological anatomy, while passingthrough different regions of healthy tissue on its way to and from thetarget region. As a result, the cumulative radiation dose in the targetregion is high and the average radiation dose to healthy tissue andcritical structures is low. Radiotherapy and radiosurgery treatmentsystems can be classified as frame-based or image-guided.

In frame-based radiosurgery and radiotherapy, a rigid and invasive frameis fixed to the patient to immobilize the patient throughout adiagnostic imaging and treatment planning phase, and a subsequenttreatment delivery phase. The frame is fixed on the patient during theentire process. Image-guided radiosurgery and radiotherapy (IGR)eliminate the need for invasive frame fixation by tracking andcorrecting for patient movement during treatment.

Image-guided radiotherapy and radiosurgery systems include gantry-basedsystems and robotic-based systems. In gantry-based systems, theradiation source is attached to a gantry that moves around a center ofrotation (isocenter) in a single plane. Each time a radiation beam isdelivered during treatment, the axis of the beam passes through theisocenter. In some gantry-based systems, known as intensity modulatedradiation therapy (IMRT) systems, the cross-section of the beam isshaped to conform the beam to the pathological anatomy under treatment.In robotic-based systems, the radiation source is not constrained to asingle plane of rotation.

In image-guided systems, patient tracking during treatment isaccomplished by registering two-dimensional (2-D) intra-treatment x-rayimages of the patient (indicating where the patient is) to 2-D referenceprojections of one or more pre-treatment three-dimensional (3-D) volumestudies of the patient (indicating where the patient should be to matchthe treatment plan). The pre-treatment 3-D volume studies may becomputed tomography (CT) scans, magnetic resonance imaging (MRI) scans,positron emission tomography (PET) scans or the like.

The reference projections (reference images), known as digitallyreconstructed radiographs (DRRs) are generated using ray-tracingalgorithms that replicate the geometry of the intra-treatment x-rayimaging system to produce images that have the same scale as theintra-treatment x-ray images. Typically, the intra-treatment x-raysystem is stereoscopic, producing images of the patient from twodifferent points of view (e.g., orthogonal views).

As x-ray imaging technology advances, the sensitivity of the x-raydetectors used to capture the intra-treatment x-ray images isincreasing. These increases are due, at least in part, to improvedimaging materials (e.g., amorphous silicon) and image capturetechnologies (e.g., CCD and CMOS imaging arrays) and processingalgorithms which reduce the quantum noise and electronic noise levels ofthe x-ray detectors and increase the signal-to-noise ratios of theintra-treatment x-ray images for any given imaging radiation level.Generally, a higher signal-to-noise ratio produces higher quality imagesthat translate to improvements in image registration and patienttracking due to improved detectability of anatomical features and/orfiducial markers. For any given noise figure, the detectability of ananatomical object can be improved by changing x-ray properties. Two suchchanges can involve increasing the imaging radiation dose or energy toincrease the SNR. FIG. 1 illustrates the improved detectability of ananatomical object 10 in a field of view 20 as the SNR is increased from1:1 to 2:1 to 5:1 as the radiation dose is increased. X-ray sources usedto generate intra-treatment x-ray images are typically set to dose andenergy levels sufficient to penetrate larger patients and provide therequired x-ray image quality (SNR level) for consistent and reliabletracking of patient and anatomical motion during setup and treatment.However, above a certain minimum SNR (e.g., 1:1), improvements inpatient tracking and image registration may be offset by increased risksto the patient from higher radiation doses.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example,and not by limitation, in the figures of the accompanying drawings inwhich:

FIG. 1 illustrates x-ray detection as a function of signal to noiseratio.

FIG. 2A illustrates an image-guided robotic radiosurgery system in oneembodiment;

FIG. 2B illustrates non-isocentric radiation treatment in one embodimentof an image-guided radiosurgery system;

FIG. 3 is a flowchart illustrating a method in one embodiment ofadaptive x-ray control;

FIG. 4 illustrates treatment nodes in one embodiment of adaptive x-raycontrol;

FIG. 5 is a flowchart illustrating a method in another embodiment ofadaptive x-ray control; and

FIG. 6 illustrates a system in which embodiments of the presentinvention may be practiced.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forthsuch as examples of specific components, devices, methods, etc., inorder to provide a thorough understanding of embodiments of the presentinvention. It will be apparent, however, to one skilled in the art thatthese specific details need not be employed to practice embodiments ofthe present invention. In other instances, well-known materials ormethods have not been described in detail in order to avoidunnecessarily obscuring embodiments of the present invention.

The term “coupled” as used herein, may mean directly coupled orindirectly coupled through one or more intervening components orsystems. The term “x-ray image” as used herein may mean a visible x-rayimage (e.g., displayed on a video screen) or a digital representation ofan x-ray image (e.g., a file corresponding to the pixel output of anx-ray detector). The term “intra-treatment x-ray image” as used hereinmay refer to images captured at any point in time during the patientsetup or treatment delivery phase of a radiosurgery or radiotherapyprocedure, which may include times when the radiation treatment sourceis either on or off. Reference to an “x-ray image” may refer to a singleimage or a simultaneous or consecutive pair of images (as in astereoscopic imaging system as described above). The term “IGR” as usedherein may refer to image-guided radiotherapy, image-guided radiosurgeryor both. A “target” as discussed herein may be an anatomical feature(s)of a patient such as a pathological anatomy (e.g., tumor, lesion,vascular malformation, nerve disorder, etc.) or normal anatomy and mayinclude one or more non-anatomical reference structures.

Unless stated otherwise as apparent from the following discussion, itwill be appreciated that terms such as “processing,” “computing,”“determining,” “estimating,”“acquiring,” “generating” or the like mayrefer to the actions and processes of a computer system, or similarelectronic computing device, that manipulates and transforms datarepresented as physical (e.g., electronic) quantities within thecomputer system's registers and memories into other data similarlyrepresented as physical within the computer system memories or registersor other such information storage, transmission or display devices.Embodiments of the method described herein may be implemented usingcomputer software. If written in a programming language conforming to arecognized standard, sequences of instructions designed to implement themethods can be compiled for execution on a variety of hardware platformsand for interface to a variety of operating systems. In addition,embodiments of the present invention are not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement embodiments ofthe present invention.

FIG. 2A illustrates the configuration of an image-guided, robotic-basedradiation treatment system 100, such as the CyberKnife® RadiosurgerySystem manufactured by Accuray, Inc. of California. In FIG. 2A, theradiation treatment source is a linear accelerator (LINAC) 101 mountedon the end of a robotic arm 102 having multiple (e.g., 5 or more)degrees of freedom in order to position the LINAC 101 to irradiate apathological anatomy (target region or volume) with beams delivered frommany angles, in many planes, in an operating volume around the patient.Treatment may involve beam paths with a single isocenter, multipleisocenters, or with a non-isocentric approach. FIG. 2B illustratesnon-isocentric radiation treatment in one embodiment. In FIG. 2B, apathological anatomy (e.g., a tumor) 201 growing around a spinal cord(202) is treated for example, by radiation treatment beams 203, 204, 205and 206, which each intersect the pathological target volume withoutconverging on a single point, or isocenter, within the target).

In FIG. 2A, the imaging system may include x-ray sources 103A and 103Band x-ray detectors 104A and 104B. The two x-ray sources 103A and 103Bmay be mounted in fixed positions on the ceiling of an operating roomand may be aligned to project imaging x-ray beams from two differentangular positions (e.g., separated by 90 degrees) to intersect at amachine isocenter 105 (which provides a reference point for positioningthe patient on a treatment couch 106 during treatment) and to illuminateimaging planes of respective x-ray detectors 104A and 104B after passingthrough the patient. In other embodiments, system 100 may include morethan two x-ray sources and more than two x-ray detectors, and any of thex-ray sources and x-ray detectors may be movable rather than fixed. Inyet other embodiments, the positions of the x-ray sources and the x-raydetectors may be interchanged or located on opposing walls.

The x-ray detectors 104A and 104B may be fabricated from a scintillatingmaterial that converts the x-rays to visible light (e.g., amorphoussilicon), and an array of CMOS (complementary metal oxide silicon) orCCD (charge-coupled device) imaging cells that convert the light to adigital image that can be compared with the reference images during theregistration process.

As noted above, the imaging radiation levels (output levels) ofconventional systems may be based on worst-case assumptions aboutimaging conditions. In one embodiment of the invention, the outputlevels of the x-ray sources (e.g., sources 103A and 103B) may beindependently adapted to actual imaging conditions and actualsignal-to-noise ratios to generate x-ray images with signal-to-noiseratios that are high enough for adequate patient tracking performancewhile minimizing a patient's x-ray exposure during setup and treatment.Initially, the x-ray sources may be set to a nominal output level based,for example, on a worst case assumption regarding signal attenuationthrough the patient or estimated from patient physiological data (e.g.,weight). Alternatively, the output level may be estimated usingattenuation data from a pre-treatment diagnostic x-ray study of thepatient (e.g., a CT scan). The SNR of an x-ray image and/or images thusacquired may be measured (see, e.g., M. J. Tapiovaara & M. Sandborg,Evaluation of Image Quality in Flouroscopy by Measurements and MonteCarlo Calculations, 40 Phys. Med. Biol. 589-607 (1995)) and used foradaptively controlling subsequent x-ray image acquisitions. The x-rayradiation properties can be optimized for each source and detector pairseparately to achieve satisfactory imaging performance while minimizingx-ray radiation exposure to the patient during setup and treatment.

FIG. 3 illustrates a method of adaptive x-ray control 300 in oneembodiment of the invention. As illustrated in FIG. 3, the method mayinclude measuring the SNR of an x-ray image generated with an x-raysource (e.g., x-ray source such as x-ray sources 103A, 103B) using aninitial output value as described above (step 301). Then, the method mayadaptively adjust the output level of the x-ray source, based on themeasured SNR (step 302). In one embodiment, step 302 may includecomparing the measured SNR with a predetermined lower SNR threshold(SNR_(L)) required for reliable patient tracking, and with apredetermined upper SNR threshold (SNR_(U)) (step 302 a). If themeasured SNR ratio is less than SNR_(L), then the method increases theoutput of the x-ray source to obtain a SNR between SNR_(U) and SNR_(L)(step 302 b). If the measured SNR is greater than SNR_(U), then themethod decreases the output of the x-ray source to obtain a SNR betweenSNR_(U) and SNR_(L). (step 302 c). If the measured SNR is betweenSNR_(U) and SNR_(L) then no change in x-ray output (or imagingparameter) is made. The output of an x-ray source may include both anenergy level and an exposure duration (on-time), and one or bothparameters may be increased or decreased to adjust the output of thex-ray source. In one embodiment, as described in greater detail below,the method may also include adjusting the output of the x-ray source forsubsequent x-ray images, in response to patient movement, to maintain aSNR between SNR_(U) and SNR_(L). (step 303). In other embodiments,described in greater detail below, subsequent x-ray images may beacquired with an image acquisition process that adapts to patientmovement (step 304). Other embodiments include apparatus, systems andarticles of manufacture capable of performing the method, described ingreater detail below.

If the measured SNR is below SNR_(L) or above SNR_(U), the system maynotify the operator of the image-guided radiation treatment system andmay request the operator to increase or decrease the output of the x-raysource to obtain a SNR within the desired SNR range. Alternatively, theadjustments to the output may be made automatically by the system (e.g.,using closed-loop feedback as is known in the art) without operatorintervention. The adjustment of the output of the x-ray source, asdescribed above, may be implemented throughout a radiation treatmentsession in response to changes in imaging conditions due to changes inpatient position resulting from intentional patient repositioning (e.g.,as part of the treatment plan) or from unplanned movement initiated bythe patient. Other imaging parameters may also be adjusted in responseto patient movement. For example, time intervals between subsequentx-rays as well as the number of x-rays may be adjusted in response topatient movement as described in greater detail below.

A radiation treatment plan, as described above, may include theapplication of radiation treatment beams to the pathological anatomyfrom a number of treatment nodes, with one or more beams being appliedfrom each node. FIG. 4 illustrates how LINAC 101 may be positioned at anode 401, which may be part of an approximately spherical distributionof nodes, for example, such as node 401. The particular number of nodesand the number of treatment beams applied at each node may vary as afunction of the location and type of pathological anatomy (for example,the number of nodes may vary from 50 to 300 and the number of beams mayvary from 200 to 1200), and may be determined during the treatmentplanning phase. A treatment plan may include target radiation doses topathological anatomies (e.g., minimum doses) and to healthy tissues andstructures (e.g., maximum doses). The total radiation dose received by apatient is the sum of the radiation dose from the treatment beams andthe radiation dose from the imaging x-ray sources. Therefore, thetreatment plan may set the total dose of the treatment beams (e.g.,number, energy level and/or duration) to account for the radiation dosefrom the planned imaging x-ray exposure (e.g., number of images, energylevel and duration).

In one embodiment, an initial treatment plan may call for x-ray imagesto be taken at fixed time intervals during the treatment (every twoseconds, for example). The initial treatment plan may be based on anexpected amount of patient movement, based for example on astatistically significant historical sample of patients undergoing thesame or similar procedure. During treatment, patient movement may bedetected by comparing two or more sequential x-ray images. Differencesbetween the sequential images may be used to register a patientcoordinate system with a treatment coordinate system to insure that thetreatment beams are accurately positioned with respect to thepathological anatomy. Differences between the sequential images may bemeasured using methods known in the art such as feature recognition,pattern intensity matching and the like (see, e.g., G. P. Penney & J.Weese, A Comparison of Similarity Measures for Use in 2D-3D MedicalImage Registration, 17 IEEE Trans. Med. Imag. 586-595, (1998)). If thedifferences between the sequential x-ray images indicate a small amountof patient movement that is well within treatment tolerances (smalldisplacement and low displacement variance), the time intervals betweensubsequent x-ray images may be increased by the image acquisitionprocess to reduce patient exposure to imaging x-rays. If the differencesbetween the sequential x-ray images indicate a large amount of patientmovement (large displacement and high displacement variance), the timeintervals between subsequent x-ray images may be decreased by the imageacquisition process.

FIG. 5 illustrates an imaging acquisition process 500, in one embodimentof step 305 of method 300 described above. The process begins bycomparing sequential x-ray images to detect patient movement (step 501).Then, if the patient movement indicated by the differences between thesequential x-ray images is greater than a movement and/or movementvariance threshold (movement thresholds), the time intervals betweensubsequent x-ray images may be decreased (step 502) to insure that anypatient movement between subsequent x-ray images does not exceed aspecified amount (e.g., a root mean square (RMS) displacement of 0.5mm). However, if the patient movement indicated by the differencesbetween the sequential x-ray images is less than the movement thresholdsduring that time interval, the time intervals between subsequent x-rayimages may be increased (step 503).

In a given treatment plan having a planned number of x-ray images and aplanned number of treatment nodes and/or treatment paths, the totaltreatment time will be determined by the time required to position andfire the radiation treatment source (e.g., LINAC 101) at each node.Increasing or decreasing the time intervals between sequential x-rayimages will result in a difference between the planned treatment timeand the planned imaging time. In one embodiment, therefore, the imageacquisition process may include adjusting the total number of x-rayimages in response to patient movement to maintain a correspondencebetween the treatment time and the imaging time. That is, the number ofx-ray images per treatment node and/or treatment path (e.g., the ratioof x-ray images to treatment nodes and/or treatment paths) may beadjusted in response to patient movement. As illustrated in FIG. 5, themethod 500 may include determining if the time intervals betweensubsequent x-ray images has been increased or decreased in response topatient movement (step 504). If the time intervals have been increased,then the total number of x-ray images may be decreased (step 504). Ifthe time intervals have been decreased, then the total number of x-rayimages may be increased (step 505).

For a given output level from an x-ray source, increasing the totalnumber of x-ray images will increase the cumulative radiation exposureduring the radiation treatment session. Conversely, decreasing the totalnumber of x-ray images will decrease the cumulative radiation exposureduring the radiation treatment session. In one embodiment, if the totalnumber of x-ray images is decreased, the method 500 may includeincreasing the treatment radiation dose in the current or a futuretreatment session (treatment fraction) to compensate for the reducedimaging x-ray exposure and maintain a planned cumulative radiationexposure (step 506). If the total number of x-ray images is increased,then the method 500 may include decreasing the treatment radiation dosein the current or a future treatment fraction to compensate for theincreased imaging x-ray exposure to maintain the planned cumulativeradiation exposure (step 507).

Motion detection using sequential x-ray images to adjust radiationexposure, as described above, may not be sufficient to detect suddenpatient movements such as those due to muscle twitches or spasms. Forexample, even if the frequency of the x-ray images is one per second, acough or muscle twitch could cause enough movement in the patient tomake the x-ray treatment beam miss its intended target. In oneembodiment, the method 500 may include a step for detecting suddenmovements, turning off the treatment x-ray source if the patientmovement exceeds a specified rate and triggering a new x-ray image (step508). Such a step may be implemented, for example, by monitoring thepatient with a machine vision system or a strain gauge attached to thepatient. Such monitoring methods are known in the art and, accordinglyare not described in detail. In one embodiment, a scanning laser maycontinuously map and monitor the 3D patient surface. Comparisons ofsequential scans can detect sudden movements that result in asignificant displacement (e.g. >1 mm), suspend treatment delivery untilpatient and target alignment can be verified, and can trigger the x-rayimage acquisition process. This approach permits a more aggressivereduction of image acquisition (as a function of time or treatment node)because significant changes in patient alignment will be detected inparallel with the sequential imaging method and trigger a new x-rayimage acquisition to verify patient alignment before treatment canresume.

FIG. 6 illustrates one embodiment of systems that may be used inperforming radiation treatment in which features of the presentinvention may be implemented. As described below and illustrated in FIG.6, a system 600 may include a diagnostic imaging system 700, a treatmentplanning system 800 and a treatment delivery system 900.

Diagnostic imaging system 700 may be any system capable of producingmedical diagnostic images of a patient that may be used for subsequentmedical diagnosis, treatment planning and/or treatment delivery. Forexample, diagnostic imaging system 700 may be a computed tomography (CT)system, a magnetic resonance imaging (MRI) system, a positron emissiontomography (PET) system, an ultrasound system or the like. For ease ofdiscussion, diagnostic imaging system 700 may be discussed below attimes in relation to a CT x-ray imaging modality. However, other imagingmodalities such as those above may also be used.

Diagnostic imaging system 700 includes an imaging source 710 to generatean imaging beam (e.g., x-rays, ultrasonic waves, radio frequency waves,etc.) and an imaging detector 720 to detect and receive the beamgenerated by imaging source 710, or a secondary beam or emissionstimulated by the beam from the imaging source (e.g., in an MRI or PETscan).

The imaging source 710 and the imaging detector 720 may be coupled to adigital processing system 730 to control the imaging operation andprocess image data. Diagnostic imaging system 700 includes a bus orother means 735 for transferring data and commands among digitalprocessing system 730, imaging source 710 and imaging detector 720.Digital processing system 730 may include one or more general-purposeprocessors (e.g., a microprocessor), special purpose processor such as adigital signal processor (DSP) or other type of device such as acontroller or field programmable gate array (FPGA). Digital processingsystem 730 may also include other components (not shown) such as memory,storage devices, network adapters and the like. Digital processingsystem 730 may be configured to generate digital diagnostic images in astandard format, such as the DICOM (Digital Imaging and Communicationsin Medicine) format, for example. In other embodiments, digitalprocessing system 730 may generate other standard or non-standarddigital image formats. Digital processing system 730 may transmitdiagnostic image files (e.g., the aforementioned DICOM formatted files)to treatment planning system 800 over a data link 1100, which may be,for example, a direct link, a local area network (LAN) link or a widearea network (WAN) link such as the Internet. In addition, theinformation transferred between systems may either be pulled or pushedacross the communication medium connecting the systems, such as in aremote diagnosis or treatment planning configuration. In remotediagnosis or treatment planning, a user may utilize embodiments of thepresent invention to diagnose or treatment plan despite the existence ofa physical separation between the system user and the patient.

Treatment planning system 800 includes a processing device 810 toreceive and process image data. Processing device 810 may represent oneor more general-purpose processors (e.g., a microprocessor), specialpurpose processor such as a digital signal processor (DSP) or other typeof device such as a controller or field programmable gate array (FPGA).Processing device 810 may be configured to execute instructions forperforming treatment planning operations discussed herein.

Treatment planning system 800 may also include system memory 820 thatmay include a random access memory (RAM), or other dynamic storagedevices, coupled to processing device 810 by bus 855, for storinginformation and instructions to be executed by processing device 810.System memory 820 also may be used for storing temporary variables orother intermediate information during execution of instructions byprocessing device 810. System memory 820 may also include a read onlymemory (ROM) and/or other static storage device coupled to bus 855 forstoring static information and instructions for processing device 810.

Treatment planning system 800 may also include storage device 830,representing one or more storage devices (e.g., a magnetic disk drive oroptical disk drive) coupled to bus 855 for storing information andinstructions. Storage device 830 may be used for storing instructionsfor performing the treatment planning steps discussed herein.

Processing device 810 may also be coupled to a display device 840, suchas a cathode ray tube (CRT) or liquid crystal display (LCD), fordisplaying information (e.g., a 2D or 3D representation of the VOI) tothe user. An input device 850, such as a keyboard, may be coupled toprocessing device 810 for communicating information and/or commandselections to processing device 810. One or more other user inputdevices (e.g., a mouse, a trackball or cursor direction keys) may alsobe used to communicate directional information, to select commands forprocessing device 810 and to control cursor movements on display 840.

It will be appreciated that treatment planning system 800 representsonly one example of a treatment planning system, which may have manydifferent configurations and architectures, which may include morecomponents or fewer components than treatment planning system 800 andwhich may be employed with the present invention. For example, somesystems often have multiple buses, such as a peripheral bus, a dedicatedcache bus, etc. The treatment planning system 800 may also include MIRIT(Medical Image Review and Import Tool) to support DICOM import (soimages can be fused and targets delineated on different systems and thenimported into the treatment planning system for planning and dosecalculations), expanded image fusion capabilities that allow the user totreatment plan and view dose distributions on any one of various imagingmodalities (e.g., MRI, CT, PET, etc.). Treatment planning systems areknown in the art; accordingly, a more detailed discussion is notprovided.

Treatment planning system 800 may share its database (e.g., data storedin storage device 830) with a treatment delivery system, such astreatment delivery system 900, so that it may not be necessary to exportfrom the treatment planning system prior to treatment delivery.Treatment planning system 800 may be linked to treatment delivery system900 via a data link 1200, which may be a direct link, a LAN link or aWAN link as discussed above with respect to data link 1100. It should benoted that when data links 1100 and 1200 are implemented as LAN or WANconnections, any of diagnostic imaging system 700, treatment planningsystem 800 and/or treatment delivery system 900 may be in decentralizedlocations such that the systems may be physically remote from eachother. Alternatively, any of diagnostic imaging system 700, treatmentplanning system 800 and/or treatment delivery system 900 may beintegrated with each other in one or more systems.

Treatment delivery system 900 includes a therapeutic and/or surgicalradiation source 910 to administer a prescribed radiation dose to atarget volume in conformance with a treatment plan. Treatment deliverysystem 900 may also include an imaging system 920 to captureintra-treatment images of a patient volume (including the target volume)for registration or correlation with the diagnostic images describedabove in order to position the patient with respect to the radiationsource. Imaging system 920 may include any of the imaging systemsdescribed above. Treatment delivery system 900 may also include adigital processing system 930 to control radiation source 910, imagingsystem 920 and a patient support device such as a treatment couch 940.Digital processing system 930 may be configured to compare 2-Dradiographic images from imaging system 920 and/or to register 2-Dradiographic images from imaging system 920 from two or morestereoscopic projections, with digitally reconstructed radiographs(DRRs) generated by digital processing system 730 in diagnostic imagingsystem 700 and/or DRRs generated by processing device 810 in treatmentplanning system 800. Digital processing system 930 may include one ormore general-purpose processors (e.g., a microprocessor), specialpurpose processor such as a digital signal processor (DSP) or other typeof device such as a controller or field programmable gate array (FPGA).Digital processing system 930 may also include other components (notshown) such as memory, storage devices, network adapters and the like.Digital processing system 930 may be coupled to radiation source 910,imaging system 920 and treatment couch 940 by a bus 945 or other type ofcontrol and communication interface.

Digital processing system 930 may implement methods (e.g., such asmethods 300 and 500 described above) to compare images obtained fromimaging system 920 and/or to register images obtained from imagingsystem 920 with pre-operative treatment planning images in order toalign the patient on the treatment couch 940 within the treatmentdelivery system 900, and to precisely position the radiation source withrespect to the target volume.

The treatment couch 940 may be coupled to another robotic arm (notillustrated) having multiple (e.g., 5 or more) degrees of freedom. Thecouch arm may have five rotational degrees of freedom and onesubstantially vertical, linear degree of freedom. Alternatively, thecouch arm may have six rotational degrees of freedom and onesubstantially vertical, linear degree of freedom or at least fourrotational degrees of freedom. The couch arm may be vertically mountedto a column or wall, or horizontally mounted to pedestal, floor, orceiling. Alternatively, the treatment couch 940 may be a component ofanother mechanical mechanism, such as the Axum® treatment couchdeveloped by Accuray, Inc. of California, or be another type ofconventional treatment table known to those of ordinary skill in theart.

It should be noted that the methods and apparatus described herein arenot limited to use only with medical diagnostic imaging and treatment.In alternative embodiments, the methods and apparatus herein may be usedin applications outside of the medical technology field, such asindustrial imaging and non-destructive testing of materials (e.g., motorblocks in the automotive industry, airframes in the aviation industry,welds in the construction industry and drill cores in the petroleumindustry) and seismic surveying. In such applications, for example,“treatment” may refer generally to the application of a beam(s) and“target” may refer to a non-anatomical object or area.

It will be apparent from the foregoing description that aspects of thepresent invention may be embodied, at least in part, in software. Thatis, the techniques may be carried out in a computer system or other dataprocessing system in response to its processor, such as processingdevice 810, for example, executing sequences of instructions containedin a memory, such as system memory 820, for example. In variousembodiments, hardware circuitry may be used in combination with softwareinstructions to implement the present invention. Thus, the techniquesare not limited to any specific combination of hardware circuitry andsoftware or to any particular source for the instructions executed bythe data processing system. In addition, throughout this description,various functions and operations may be described as being performed byor caused by software code to simplify description. However, thoseskilled in the art will recognize what is meant by such expressions isthat the functions result from execution of the code by a processor orcontroller, such as processing device 810.

A machine-readable medium can be used to store software and data whichwhen executed by a data processing system causes the system to performvarious methods of the present invention. This executable software anddata may be stored in various places including, for example, systemmemory 820 and storage 830 or any other device that is capable ofstoring software programs and/or data.

Thus, a machine-readable medium includes any mechanism that provides(i.e., stores and/or transmits) information in a form accessible by amachine (e.g., a computer, network device, personal digital assistant,manufacturing tool, any device with a set of one or more processors,etc.). For example, a machine-readable medium includesrecordable/non-recordable media (e.g., read only memory (ROM); randomaccess memory (RAM); magnetic disk storage media; optical storage media;flash memory devices; etc.), as well as electrical, optical, acousticalor other forms of propagated signals (e.g., carrier waves, infraredsignals, digital signals, etc.); etc.

It should be appreciated that references throughout this specificationto “one embodiment” or “an embodiment” means that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention.Therefore, it is emphasized and should be appreciated that two or morereferences to “an embodiment” or “one embodiment” or “an alternativeembodiment” in various portions of this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined assuitable in one or more embodiments of the invention. In addition, whilethe invention has been described in terms of several embodiments, thoseskilled in the art will recognize that the invention is not limited tothe embodiments described. The embodiments of the invention can bepracticed with modification and alteration within the scope of theappended claims. The specification and the drawings are thus to beregarded as illustrative instead of limiting on the invention.

1. A method, comprising: measuring a signal-to-noise ratio (SNR) of anx-ray image generated with an x-ray source; and adaptively adjusting anoutput of the x-ray source based on the SNR of the x-ray image.
 2. Themethod of claim 1, wherein adaptively adjusting the output of the x-raysource comprises: increasing the output to increase the SNR if the SNRis less than a lower SNR threshold; and decreasing the output todecrease the SNR if the SNR is greater than an upper SNR threshold. 3.The method of claim 2, further comprising: adjusting the output forsubsequent x-ray images in response to patient movement to maintain theSNR between lower SNR threshold and the upper SNR threshold.
 4. Themethod of claim 2, wherein adjusting the output of the x-ray sourcecomprises adjusting at least one of an energy level of the x-ray sourceand an operating duration of the x-ray source.
 5. The method of claim 2,wherein adjusting the output of the x-ray source is performedautomatically.
 6. The method of claim 2, further comprising requestingauthorization from a system operator before adjusting the output of thex-ray source.
 7. The method of claim 2, further comprising: acquiringsubsequent x-ray images with an image acquisition process that adapts topatient movement.
 8. The method of claim 7, wherein the imageacquisition process comprises: comparing sequential x-ray images todetect patient movement; and adjusting time intervals between subsequentx-ray images in response to the patient movement.
 9. The method of claim8, wherein adjusting the time intervals comprises: increasing the timeintervals between the subsequent x-ray images when the patient movementdecreases; and decreasing the time intervals between the subsequentx-ray images when the patient movement increases.
 10. The method ofclaim 9, wherein the image acquisition process comprises: adjusting atotal number of x-ray images taken during an image-guided radiationtreatment session in response to patient movement.
 11. The method ofclaim 10, wherein adjusting the total number of x-ray images comprises:decreasing the total number of x-ray images taken during theimage-guided radiation treatment session when the time intervals betweenthe subsequent x-ray images have been increased in response to thepatient movement; and increasing the total number of x-ray images takenduring the image-guided radiation treatment session when the timeintervals between the subsequent x-ray images have been decreased inresponse to the patient movement.
 12. The method of claim 10, furthercomprising: adjusting a radiation treatment dose to compensate for achange in imaging x-ray exposure due to adjustments to the total numberof x-ray images and the output of the x-ray source in response to thepatient movement, wherein the radiation treatment dose is at least oneof a current treatment fraction and a future treatment fraction.
 13. Themethod of claim 11, further comprising: decreasing the output level ofthe x-ray source to maintain a cumulative radiation exposure level belowa cumulative radiation exposure limit when the total number of x-rayimages taken during the image-guided radiation treatment session isincreased.
 14. The method of claim 7, wherein the image acquisitionprocess comprises: adjusting a ratio of x-ray images to treatment nodesin response to patient movement.
 15. The method of claim 14, whereinadjusting the ratio comprises: decreasing the ratio of x-ray images totreatment nodes when the patient movement is less than an amount ofmovement or movement variance assumed during a treatment planning phase:and increasing the ratio of x-ray images to treatment nodes when thepatient movement is greater than the amount of movement or movementvariance assumed during the treatment planning phase.
 16. The method ofclaim 15, wherein each treatment node comprises one or more treatmentpaths, wherein the image acquisition process further comprises:adjusting a ratio of the x-ray images to the treatment paths in responseto patient movement.
 17. The method of claim 7, further comprising:detecting patient movement during an image-guided radiation treatmentsession; and triggering the acquisition of x-ray images in response topatient movement greater than a displacement that exceeds a targetingaccuracy threshold.
 18. The method of claim 17, wherein detectingpatient movement comprises at least one of: comparing sequential x-rayimages; monitoring the patient with a machine vision system; reading astrain gauge attached to the patient; and laser-scanning a 3-D contourof the patient.
 19. A system comprising: an imaging system including anx-ray source to generate x-ray images; and a processing device tocontrol the imaging system, wherein the processing device is configuredto measure a signal-to-noise ratio (SNR) of an x-ray image generatedwith an x-ray source; and adaptively adjust an output of the x-raysource based on the SNR of the x-ray image.
 20. The system of claim 19,wherein to adjust the output of the x-ray source, the processing deviceis configured to: increase the output to increase the SNR if the SNR isless than a lower SNR threshold; and decrease the output to decrease theSNR if the SNR is greater than an upper SNR threshold.
 21. The system ofclaim 20, wherein the processing device is further configured to: adjustthe output for subsequent x-ray images in response to patient movementto maintain the SNR between lower SNR threshold and the upper SNRthreshold.
 22. The system of claim 19, wherein the output of the x-raysource comprises an energy level and a duration, and wherein to adjustthe output of the x-ray source, the processing device is configured toadjust at least one of the energy level and the duration.
 23. The systemof claim 22, wherein the processing device is configured to adjust theoutput level automatically without operator intervention.
 24. The systemof claim 22, wherein the processing device is configured to obtainauthorization from an operator to adjust the output level of the x-raysource.
 25. The system of claim 20, wherein the imaging system isfurther configured, under control of the processing device to: acquiresubsequent x-ray images with an image acquisition process that adapts topatient movement.
 26. The system of claim 25, wherein the processingdevice is configured to: compare sequential x-ray images to detectpatient movement; and to adjust time intervals between subsequent x-rayimages in response to the patient movement.
 27. The system of claim 26,wherein to adjust time intervals the processing device is configured to:increase the time intervals between the subsequent x-ray images when thepatient movement decrease; and to decrease the time intervals betweenthe subsequent x-ray images when the patient movement increases.
 28. Thesystem of claim 25, wherein the processing device is further configuredto: adjust a total number of x-ray images taken during an image-guidedradiation treatment session in response to patient movement.
 29. Thesystem of claim 28, wherein to adjust the total number of x-ray imagesthe processing device is configured to: decrease the total number ofx-ray images taken during the image-guided radiation treatment sessionwhen the time intervals between the subsequent x-ray images have beenincreased in response to the patient movement; and to increase the totalnumber of x-ray images taken during the image-guided radiation treatmentsession when the time intervals between the subsequent x-ray images havebeen decreased in response to the patient movement.
 30. The system ofclaim 27, further comprising a radiation delivery system to deliver aradiation treatment dose, wherein the processing device is furtherconfigured to adjust the radiation treatment dose to compensate for achange in imaging x-ray exposure due to adjustments to the total numberof x-ray images and the output of the x-ray source in response to thepatient movement, wherein the radiation treatment dose is at least oneof a current treatment fraction and a future treatment fraction.
 31. Thesystem of claim 29, wherein the processing device is further configuredto: decrease the output level of the x-ray source to maintain acumulative radiation exposure level below a cumulative radiationexposure limit when the total number of x-ray images taken during theimage-guided radiation treatment session is increased.
 32. The system ofclaim 31, wherein the processing device is further configured to: adjusta ratio of x-ray images to treatment nodes in response to the patientmovement.
 33. The system of claim 32, wherein to adjust the ratio theprocessing device is configured to: decrease the ratio when the patientmovement is less than an amount of movement or movement variance assumedduring a treatment planning phase; and to increase the ratio when thepatient movement is greater than the amount of movement or movementvariance assumed during the treatment planning phase.
 34. The system ofclaim 33, wherein each treatment node comprises one or more treatmentpaths, and wherein the processing device is further configured to:adjust a ratio of the x-ray images to the treatment paths in response tothe patient movement.
 35. The system of claim 25, wherein the processingdevice is further configured to: detect patient movement during animage-guided radiation treatment session; interrupt the radiationtreatment and trigger the acquisition of x-ray images if the patientmovement is greater than a displacement that exceeds a targetingaccuracy threshold.
 36. An article of manufacture comprising: amachine-accessible medium including data that, when accessed by amachine, cause the machine to perform operations comprising: measuring asignal-to-noise ratio (SNR) of an x-ray image generated with an x-raysource; and adaptively adjusting an output of the x-ray source based onthe SNR of the x-ray image.
 37. The article of manufacture of claim 36,wherein the operation of adaptively adjusting the output level of thex-ray source comprises: increasing the output to increase the SNR if theSNR is less than a lower SNR threshold; and decreasing the output todecrease the SNR if the SNR is greater than an upper SNR threshold. 38.The article of manufacture of claim 37, wherein the machine-accessiblemedium further includes data that cause the machine to performoperations comprising: adjusting the output for subsequent x-ray imagesin response to patient movement to maintain the SNR between lower SNRthreshold and the upper SNR threshold.
 39. The article of manufacture ofclaim 37, wherein the operation of adjusting the output of the x-raysource comprises adjusting at least one of an energy level of the x-raysource and an operating duration of the x-ray source.
 40. The article ofmanufacture of claim 37, wherein the operation of adjusting the outputof the x-ray source is performed automatically without operatorintervention.
 41. The article of manufacture of claim 37, wherein themachine-accessible medium further includes data that cause the machineto perform operations comprising: requesting authorization from a systemoperator before adjusting the output of the x-ray source.
 42. Thearticle of manufacture of claim 37, wherein the machine-accessiblemedium further includes data that cause the machine to performoperations comprising: acquiring subsequent x-ray images with an imageacquisition process that adapts to patient movement.
 43. The article ofmanufacture of claim 42, wherein the operation of acquiring subsequentx-ray images comprises: comparing sequential x-ray images to detectpatient movement; and adjusting time intervals between the subsequentx-ray images in response to the patient movement.
 44. The article ofmanufacture of claim 43, wherein the operation of adjusting timeintervals between the subsequent x-ray images in response to the patientmovement comprises: increasing the time intervals between the subsequentx-ray images when the patient movement decreases; and decreasing thetime intervals between the subsequent x-ray images when the patientmovement increases.
 45. The article of manufacture of claim 44, whereinthe machine-accessible medium further includes data that cause themachine to perform operations comprising: adjusting a total number ofx-ray images taken during an image-guided radiation treatment session inresponse to the patient movement.
 46. The article of manufacture ofclaim 45, wherein the operation of adjusting the total number of x-rayimages taken during the image-guided radiation treatment session inresponse to the patient movement comprises: decreasing the total numberof x-ray images taken during the image-guided radiation treatmentsession when the time intervals between the subsequent x-ray images havebeen increased in response to the patient movement; and increasing thetotal number of x-ray images taken during the image-guided radiationtreatment session when the time intervals between the subsequent x-rayimages have been decreased in response to the patient movement.
 47. Thearticle of manufacture of claim 45, wherein the machine-accessiblemedium further includes data that cause the machine to performoperations comprising: adjusting a radiation treatment dose tocompensate for a change in imaging x-ray exposure due to adjustments tothe total number of x-ray images and the output of the x-ray source inresponse to the patient movement, wherein the radiation treatment doseis at least one of a current treatment fraction and a future treatmentfraction.
 48. The article of manufacture of claim 46, wherein themachine-accessible medium further includes data that cause the machineto perform operations comprising: decreasing the output level of thex-ray source to maintain a cumulative radiation exposure level below acumulative radiation exposure limit when the total number of x-rayimages taken during the image-guided radiation treatment session isincreased.
 49. The article of manufacture of claim 42, wherein themachine-accessible medium further includes data that cause the machineto perform operations comprising: adjusting a ratio of x-ray images totreatment nodes in response to the patient movement.
 50. The article ofmanufacture of claim 49, wherein the operation of adjusting the ratio ofx-ray images to treatment nodes in response to the patient movementcomprises: decreasing the ratio of x-ray images to treatment nodes whenthe patient movement is less than an amount of movement or movementvariance assumed during a treatment planning phase; and increasing theratio of x-ray images to treatment nodes when the patient movement isgreater than the amount of movement or movement variance assumed duringthe treatment planning phase.
 51. The article of manufacture of claim50, wherein each treatment node comprises one or more treatment paths,and wherein the machine-accessible medium further includes data thatcause the machine to perform operations comprising: adjusting a ratio ofthe x-ray images to the treatment paths in response to patient movement.52. The article of manufacture of claim 42, wherein themachine-accessible medium further includes data that cause the machineto perform operations comprising: detecting the patient movement duringthe image-guided radiation treatment session; interrupting theimage-guided radiation treatment session and triggering the acquisitionof x-ray images if the patient movement is greater than a displacementthat exceeds a targeting accuracy threshold.
 53. The article ofmanufacture of claim 52, wherein the operation of detecting patientmovement during the image-guided radiation treatment session comprisesat least one of: comparing sequential x-rays; monitoring the patientwith a machine vision system; reading a strain gauge attached to thepatient; and laser-scanning a 3-D contour of the patient.
 54. Anapparatus, comprising: means for measuring a signal-to-noise ratio (SNR)of an x-ray image generated with an x-ray source; and means foradaptively adjusting an output of the x-ray source based on the SNR ofthe x-ray image.
 55. The apparatus of claim 54, wherein the means foradaptively adjusting the output level of the x-ray source comprises:means for increasing the output to increase the SNR if the SNR is lessthan a lower SNR threshold; and means for decreasing the output todecrease the SNR if the SNR is greater than an upper SNR threshold. 56.The apparatus of claim 55, further comprising: means for adjusting theoutput for subsequent x-ray images in response to patient movement tomaintain the SNR between lower SNR threshold and the upper SNRthreshold.
 57. The apparatus of claim 55, further comprising: means foracquiring subsequent x-ray images with an image acquisition process thatadapts to patient movement.